Why would you use a wire that was previously unused? That would not be wise if you are not familiar with the control voltage. However if you bought a home depot cheap t-stat that does not use a common wire and is battery operated then you could wire just like the old one, however if the new one needs a common wire then chances are you would have to connect said wire at the terminal board inside of the air handler. Don't mess it up though you could fry the transformer but don't worry they do make new ones to replace them .

Leave well enough alone! What made you go digging into the panel anyway? Heatseeker is right in that the blue wire MAY have been used as a common connection for a thermostat powered from the airhandler/furnace. If you every change thermostats to one that needs 24 volts power at the thermostat then you have that extra wire, if needed. And if you ever do any work, always, always, turn off all power to save your electronics and, oh yes, yourself.

if so, how many wires are connected and what are the colors and terminal identifications ? also, look at the diagram in the box with the thermostat. it should show you whether you need a 'common'. the 'r' terminal receives 24 volts from the furnace transformer. that 24 volts is distributed on 'g', 'y' and 'w' back to the furnace to turn on the devices. the 'common' in question, usually labelled 'c', is to provide a path back to the return side of the power supply (transformer). this is only required if the thermostat itself needs a power supply. the diagram should tell you whether it is required or optional. if 'c' was not used before, the extra wire can be used if the thermostat requires it. let us know what you see in the instructions and if it is needed, you will need to connect the other end at the furnace. we may be able to direct you how to do this if you are capable and confident.